In prior art the control components, like for example direction and flow control valves, used in control of fluid motors subjected to loads, respond to manual, electrical or other remote control signals, at a comparatively low energy level, by proportionally amplifying such signals for transmittal to the control elements of the system, though the use of fluid power energy. Such fluid power energy can be derived from a separate source of fluid power, or from the main system pump, which powers the hydraulic system.
The use of a separate source of fluid power to provide the energy for operation of the system controls is very desirable, since it is completely independent of the duty cycle of the primary hydraulic power system. However, such an independent source of pressure suffers from several disadvantages, like for example inefficient use of power, especially with the hydraulic system in standby condition. Also in mobile type systems, using internal combustion engines as the prime mover, such a separate source of fluid power needs a separate power takeoff, which is not only expensive, but also utilizes a lot of space, which in such applications is at a premium.
Many industrial and mobile type systems use fluid flow at system pressure derived from the main system pump. In such systems, especially during the control of negative loads, the system pressure, which is dictated by the magnitude of the load, may drop to a pressure level below that required by the system controls. This disadvantage can be overcome by preventing the discharge pressure of the system pump dropping below a certain minimum pressure level, as dictated by the characteristics of the system controls. Some of those controls, especially of an electro-hydraulic servo type, well known in the art, require a relatively high pressure level, which results in the loss of large amounts of fluid power in the main fluid power system, especially when controlling small positive loads, or negative type loads.
Another disadvantage of this approach results from the trend, well known to those skilled in the art, of using in mobile systems maximum operating system pressures, well in excess of say 5000 PSI. In such systems not only pressure reducing type devices must be interposed between the main system pump and system controls, but the use of say 5000 PSI pressure, at substantially high flow levels, to supply fluid power to system controls, at say 1000 PSI pressure level, results in large amounts of fluid power being converted to heat by throttling, loss in system efficiency and loss of power derived from tee system pump to perform the work at the tool.